(1) Field of the Invention
This invention relates to a paste-type cadmium electrode for use in an alkaline storage cell such as a nickel-cadmium cell and its manufacturing method.
(2) Description of the Prior Art
As a cadmium negative electrode for use in an alkaline storage cell like a nickel-cadmium storage cell, a paste-type cadmium non-sintered negative electrode is widely used in the industrial field because it is manufactured easily and at a low cost. This type of cadmium electrode is produced by, for example, kneading an active material such as powdered cadmium oxide or powdered cadmium hydroxide with a binding solution to form a paste, coating the paste on a conductive substrate and drying it. However, this type of cadmium electrode has a problem of low absorption of oxygen gas, and various proposals have been made to solve this problem.
For example, Japanese Patent Publication laid-open Nos. 60-216449 and 61-240576 have proposed forming a conductive layer comprising carbon or an alkali-resistant conductive substance such as nickel on a surface of a paste-type cadmium electrode plate. This method increases the conductivity of the surface of the cadmium electrode plate and promotes formation of metal cadmium on the surface, whereby heightening the oxygen gas absorbing ability of the electrode plate.
However, the above-attained oxygen gas absorbing ability is not enough, and moreover other problems occur when an alkali-resistant substance is used to form a conductive layer.
The reason why the above-attained absorbing ability is not enough will be explained hereinafter. When a conductive layer comprising powdered carbon is formed on the surface of the paste-type cadmium electrode plate, the metal cadmium formation is promoted on an interface between the conductive layer and the active material layer during charging, whereby the oxygen gas absorbing ability is improved. However, the metal cadmium formation is not promoted inside the conductive layer or on its surface, where metal cadmium and oxygen gas are most easily reacted with each other. Therefore, the cadmium electrode plate does not have enough oxygen gas absorbing ability.
When a conductive layer comprising an alkali-resistant substance such as powdered nickel is formed on the surface of the paste-type cadmium electrode plate, the metal cadmium formation is promoted inside and on the surface of the conductive layer during charging due to the high conductivity of the conductive substance. As a result, the oxygen gas absorbing ability of the electrode plate is further improved than when powdered carbon is used. However, this method involves other problems of promotion of hydrogen gas and dendrite generation and decrease of the cell storage characteristic.
Another proposal, which is a combination of the above two, is forming a conductive layer comprising a mixture of powdered carbon and a powdered alkali-resistant metal on the surface of the paste-type cadmium electrode plate. This method cannot solve the above problems either if powdered nickel only or the like is used as the powdered alkali-resistant metal.
Apparently from the above, it is indispensable to modify the conditions of the powdered alkali-resistant metal inside the conductive layer to solve the above problems while maintaining the oxygen gas absorbing ability.
Usually, the apparent density of a powdered alkali-resistant metal is 0.2 g/cc or more (0.5 g/cc in the case of carbonyl nickel). This means it is almost impossible to uniformly disperse the powdered alkali-resistant metal into a slurry comprising powdered carbon and a binder. Accordingly, it is also hardly possible that a conductive layer which is produced by coating the slurry on the surface of the electrode plate and drying it has the powdered alkali-resistant metal uniformly dispersed therein.
Moreover, such alkali-resistant metals as nickel and iron are low in hydrogen overvoltage than a compound such as cadmium oxide acting as an active material. Therefore, if the conductive layer has too high a conductivity, hydrogen gas may be generated on the liquid-solid interface during charging. This means the form and range of amount of nickel should be determined.
If powdered nickel is used as a powdered alkali-resistant metal, each nickel grain is contacted with the active material at a point. Since the metal cadmium produced during charging reaches the surface of the electrode plate, only the nickel existing on the surface of the electrode plate contributes to the improvement of conductivity. If too much nickel is added to utilize as much nickel as possible, the electrode plate has high enough a conductivity to generate hydrogen gas.
If the surface of the paste-type cadmium electrode plate is nickel-plated, the surface is covered with a nickel layer to result in promoting hydrogen gas generation and in preventing electrode reaction.
Adding a fiber-type metal to the conductive layer is effective in forming a network of metal cadmium on the surface of the paste-type cadmium electrode plate. However, since the fiber diameter of fiber-type nickel is as large as several to tens of several micron meters and so the fibers are poor in flexibility, the fibers are pierced into the separator and possibly occur shortcircuiting of the cell.
Usually in producing such a cadmium negative electrode plate, electrochemical formation for conducting reserve charging amount is eliminated and thus simplify the manufacturing procedure by mixing metal cadmium produced by reserve charging to the active material paste including cadmium oxide or the like. However, metal cadmium is low in utilization factor of the active material in general. Japanese Patent Publication laid-open No. 62-243254 has proposed using metal cadmium including indium in order to stimulate the reaction with metal cadmium. Even this method cannot increase the utilization factor enough while solving the above problems.